In view of the great deal of work showing the production and efficient repair or cyclobutyldimers in normal cells, it becomes vital to evaluate the production and repairability of other UV-induced products, which if inefficiently repaired could play a large or even major role in cellular UV sensitivity. These photoproducts are pyrimidine adducts (Pyr(6-4)Pyo or Pyr(5-4)Pyo), pyrimidine coupled products (Pyr(5-5)Pyr), pyrimidine photohydrates (ho6hUra), and a "spore" product (Thy (alpha-5)hThy). Progress has been made with all these areas. Analysis of absorption and emission spectra was made in order to understand the excited state processes and solution conformation of eight diphyrimidine adducts. Polarographic study of electroreduction of Cyt(5-4)Pyo and Cyd(5-4)Pdo showed that both involve the same redox pattern. The reaction involves simultaneous addition of an electron and a proton to form a free radical that rapidly dimerizes. 5,5'-Diuridinyl was identified as a major product from UV-irradiation of polynucleotides containing bromouracil. The relevance of this type of coupled product (Urd(5-5)Urd) to the increased radiation sensitivity of DNA should be scrutinized. C(5)-Photoexchange has been shown to exist as a second major excited singlet reaction of cytidine and it competes with the photohydration reaction. BIBLIOGRAPHIC REFERENCES: William W. Hauswirth, and Shih Yi Wang, "Cytidine-C(5)-Photoexchange. A Photokinetic Analysis". Protochem. and Photobiol., 1976 (in press). William W. Hauswirth, and Shih Yi Wang, "Excited State Processes and Solution Conformation of Pyrimidine Adducts". Photochem. and Photobiol., 1976 (in press).